A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Important factors for the performance of a lithographic apparatus are the throughput, i.e. the number of wafers that is produced within a certain period, and the overlay, i.e. the production quality. In industry, there is a continuous demand to improve the throughput and overlay of the lithographic apparatus.
In the known lithographic apparatus, the substrate stage accuracy, which is measured in 6 degrees of freedom and is important for overlay, is controlled by using a combination of single input single output (SISO) feedback and feed-forward control for each of the 6 axes. The feedback controller guards (robust) stability and increases disturbance rejection, while the feed-forward controller improves tracking performance.
Generally the two demands higher throughput and overlay performance, contradict each other; higher accelerations (and jerk) cause higher internal dynamic vibrations (or deformations) of the stages, which result in a deterioration of the substrate stage accuracy.